A lithium ion secondary battery has been developed as a secondary battery, which can achieve high voltage and high energy density.
The main components thereof are positive and negative electrodes, and an ion conductive layer disposed between these electrodes. It is possible to obtain a battery, which can be charged and discharged in a larger amount of current by decreasing the resistance with an ion conductive layer. Therefore, an ion conductive layer is required to decrease ion conductive resistance, and the electrodes must be arranged with an appropriate distance therebetween to prevent short-circuit.
In lithium ion secondary batteries in practical use, as disclosed in Japanese Unexamined Patent Publication No. 8-83608 (1996), a porous separator film is used to separate the electrodes from each other by an appropriate distance, and an electrolytic solution for ion conduction is charged between these two electrodes to realize ion migration between them.
A lithium ion secondary battery in practical use has a structure, wherein the above-mentioned components are stored in a solid outer can comprising metal or the like.
A solution obtained by dissolving a lithium salt into a mixed solvent of a main solvent and a sub solvent is employed as the electrolytic solution for the above-mentioned lithium ion secondary battery of prior art, and lithium hexafluorophosphate (LiPF.sub.6) is employed as a salt of the electrolytic solution, since the solution and the salt bring about extremely high ion conductivity and high electrochemical stability. Since a non-aqueous electrolytic solution having inferior ion conductivity relative to an aqueous solution needs to be employed in a lithium ion secondary battery, the above mentioned salts securing high ion conductivity are employed without exception.
However, LiPF.sub.6 has a disadvantage of thermal unstability, as it has been pointed out that a LiPF.sub.6 type electrolytic solution begins to decompose at a lower temperature than an electrolytic solution containing a salt such as LiBF.sub.4 through the decomposing reaction shown in the following chemical reaction formula (1). EQU LiPF.sub.6.fwdarw.PFs(gas)+LiF (1)
In the publication 1 (ELECTROCHEMISTRY AND INDUSTRIAL PHYSICAL CHEMISTRY, 65, No. 11, pp. 900-908, 1997), it is described that PF.sub.5 formed by the above decomposition reaction is a gas itself, and a decomposition gas is evolved by reacting with solvent molecules. The formation of the gas inside the battery may cause deformation or breakage of the battery outer can.
For this reason, from the viewpoint of a battery design, it is necessary to use a solid outer can, which is difficult to deform or break. But a solid outer can is not preferable from the viewpoint of a battery energy density because of its heavy weight and large volume.
Moreover, as the decomposition reaction shown in the formula (1) proceeds, the concentration of LiPF.sub.6 undesirably decreases, which reduces ion conductivity of an electrolytic solution.
The present invention has been contrived to solve these problems, and an object of the present invention is to provide a battery electrolytic solution having excellent stability, and also to provide a battery with excellent battery characteristics and a lightweight outer structure.